1. Field of the Invention
The present invention relates to impurity doping methods for semiconductor devices, and particularly to manufacturing methods of semiconductor devices by annealing methods using high-intensity light sources.
2. Description of the Related Art
Improvements in the performance of large-scale integration (LSI) semiconductor devices can be realized by miniaturizing, or put more plainly, by miniaturizing the elements that compose a semiconductor device. Thus, LSI becomes increasingly more integrated while miniaturization of elements such as metal-oxide-semiconductor (MOS) transistors is being taken to a whole new level. Along with the miniaturization of composing elements, parasitic resistance and short channel effects on-MOS transistors and the like, increase. Thus there is increased importance placed on the formation of low resistance layers and shallow pn junctions.
For forming a shallow pn junction with a thickness of or below twenty nm, a thin impurity doped region is formed using an ion implantation in a semiconductor substrate with low acceleration energy. The impurities doped in the semiconductor substrate are activated by an annealing process, thus forming a shallow impurity diffusion region. In order to lower layer resistance of an impurity diffusion region it is necessary to perform activation annealing of the impurities at a high temperature.
However, the diffusion coefficients of p-type impurities such as boron (B), and n-type impurities such as phosphorus (P) or arsenic (As), in the crystal of the silicon (Si) substrate, are large. In the time needed to perform rapid thermal annealing (RTA) using current halogen lamps, impurities diffuse to both the interior and exterior of a semiconductor substrate. As a result, it is impossible to form a shallow impurity diffusion region having a high concentration of impurities on a semiconductor substrate. Also, it becomes impossible to activate a high concentration of impurities if the temperature of the RTA process is lowered in order to control the diffusion of the impurities. In this manner, it is difficult to form a shallow impurity diffusion region having low resistance and a high concentration of activated impurities.
Impurities such as indium (In) and antimony (Sb) are being tested for use in further miniaturization of semiconductor devices. Compared to impurities such as B, P, and As, In and Sb have higher atomic masses, to achieve a more precipitous impurity distribution by ion implantation at the same acceleration energy. However, the solid solubility limit of substances such as In in Si crystal is low. It becomes necessary to raise annealing temperatures of RTA while further prolonging annealing processing time in order to activate In impurities implanted by ion implantation. As a result, it is impossible to maintain precipitous impurity distribution. Recently a pulse light annealing method using pulse light sources such as a flash lamp or a YAG laser, which can instantly supply the energy essential to impurity activation, is being tested as a solution to the RTA problem. A xenon (Xe) flash lamp has a quartz glass tube filled with Xe gas, in which electrical charges stored in capacitors and the like, are instantaneously discharged. As a result, it becomes possible to illuminate a high intensity white light in a range of several hundred μs to several hundred ms for instance. It is possible to attain heat energy required for impurity activation in the instantaneous heating of a semiconductor substrate absorbing flash lamp light. Therefore, it is possible to activate a high concentration of impurities while leaving the concentration profile of the impurities implanted into the semiconductor substrate virtually unchanged.
Providing a low resistance gate electrode is important in the miniaturization of transistors. In gate electrodes that use polycrystalline silicon (simplified to poly-Si hereinafter), impurities are ion implanted into the gate electrode during the formation of the source-drain regions. The ion implanted impurities are activated by annealing, and diffused throughout the entire gate electrode. However, the period of time used for annealing on flash lamp annealing methods is short, and diffusion of impurities that have been ion implanted into the gate electrode is suppressed. As a result, a poly-Si layer that has a low carrier concentration remains within a gate electrode. A poly-si layer that has a low carrier concentration will deplete a gate electrode. Depletion of a gate electrode increases the effective thickness of a gate insulating film, and invites a drop in the current driving force of a transistor. Stated plainly, by current flash lamp annealing technology, even though an impurity diffusion region having a shallow junction with low resistance is formed, fabrication of a high performance miniature transistor remains a difficulty.